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I. Introduction
The solar system—our Sun's system of planets, moons, and smaller debris—is humankind's cosmic backyard. Small by factors of millions compared to interstellar distances, the spaces between the planets are daunting but surmountable stepping stones toward the human dream of interstellar flight. And it is within this cosmic backyard that the immediate clues to our own origin—that of life, and of the Earth as a persistently habitable world—are to be found. We wonder, as we look up at our neighboring planets on a dark, moonless night, whether life is to be found on these worlds, either viable communities of simple organisms or remains that have been dead for geologically-long periods of time. If so, then perhaps the universe beyond our backyard is teeming with life, from the simple to the complex. If, instead, we find our planetary neighbors to be sterile testaments to a delicate fine-tuning of conditions necessary for initiating and sustaining life, then we must ask ourselves whether we are alone in a vast, impersonal cosmos.
It is for these reasons that we explore the solar system with robotic emissaries: to flex our technological muscle by crossing vast distances and operating in exotic and extreme environments; to understand how the planets came to be and what triggered different evolutionary paths among worlds; to trace the early history of our own planet Earth and how it came to be habitable; to search for evidence of extinct or extant life and life's precursory chemistry on and within neighboring planetary bodies. Mars is an important target of these endeavors but not the only one; were the red deserts and canyons of that world to be our only goal, humanity's explorations beyond Earth would be greatly impoverished. Likewise the Moon, despite its importance as a signpost of the first billion years of Earth's history, is no more than a stepping-stone to a surprising array of vastly different and more complex planetary worlds that lie beyond. We must explore the solar system in its vastness and variety; we must commit as the Earth's most advanced spacefaring nation to extending humankind's reach across an almost daunting array of different worlds. We must explore!
The United States has committed itself to the continued exploration of the solar system through the President's "Moon, Mars and Beyond" initiative. As a result of this initiative, it is an agency goal to "Conduct robotic exploration across the solar system for scientific purposes and to support human exploration. In particular, explore the moons of Jupiter, asteroids, and other bodies to search for evidence of life, to understand the history of the solar system, and to search for resources."
But how do we construct an economically rational and technologically achievable ordering of planetary targets and exploration? The approach suggested in this roadmap begins with a set of five "scientific objectives":
- Learn how the sun's family of planets and minor bodies originated
- Determine how the solar system evolved to its current diverse state including the origin and evolution of the Earth's biosphere
- Explore the space environment to discover potential hazards and search for resources that would enable permanent human presence
- Understand the processes that determine the fate of the solar system and life within it
- Determine if there is or ever has been life elsewhere in the solar system
The five objectives can be understood as addressing, in different ways, the fundamental goal of understanding how our solar system became, and planetary systems in general become, habitable—and how they maintain that ability to nurture life. How do planets that can support life arise, and what is the probability that any given system will have a habitable planet? Scientific objective 1 addresses the goal through a deeper understanding of the mechanisms by which our solar system formed, and whether our own system is a typical or unusual outcome of the general process of planetary system formation. Scientific objective 2 seeks to quantify how the planets and the space environment surrounding them evolved to the state we see today, and how this evolution affected the capability of particular planetary environments to nurture life. Scientific objective 3 addresses habitability through the present day space environment, the hazards that it presents in the near-future to Earthly life, and the potential opportunities it provides through resources to support the spread of humankind throughout the solar system. Scientific objective 4 stimulates exploration of planetary neighbors whose current environments are uninhabitable, and whose evolutionary history in arriving there might presage aspects of the future evolution of our own, currently habitable, home world. Finally, the search for life or evidence of past life elsewhere in the solar system is embodied in scientific objective 5—a mandate to understand whether Earth is and has always been the only habitable planet in our solar system.
Habitability, then, is the key word that drives the strategy in the program of exploration laid out here. But the question of habitability must be parsed, from a practical standpoint, into two threads that lead more directly to a prioritization of targets and exploration objectives. The first thread is that of habitability in planetary environments: how have specific planetary environments evolved with time, when and in what way were they habitable, and does life exist there now? The second thread is habitability associated with planetary system architecture: what determines the arrangements of planetary systems, what roles do the positions and masses of giant planets play in the formation of habitable planets and moons and the delivery to them of the chemical ingredients of life, and how have our own giant planets shaped the evolution of the impact hazard population in our own system? Both threads speak to the fundamental issue of how planetary systems become habitable by exploring our own solar system from two complimentary perspectives—comparative exploration of worlds, and exploration of planetary architecture. Both threads connect to other strategic roadmaps through the exploration of Mars as a once habitable world, and the exploration of the Moon as a preserved record of the earliest evolution of the Earth and its impact environment. And both connect to the compelling question, encapsulated in a third roadmap, of the potential variety and habitability of planetary systems around other stars.
Both threads require a mixture of small-, medium-, and large-class missions. The small ($300-500M) missions, carried out through the Discovery Program, are PI-led missions that allow fast response to address a specific set of high value scientific questions at targets that may be less technically challenging. For this reason, Discovery will pay a crucial role, as described below, in the exploration of small bodies—asteroids, comets—that provide key clues to the chemistry of solar system formation, impact hazards through time, and the shaping of the architecture of our own planetary system.
Medium-class ($500-800M) missions in solar system exploration, New Frontiers, are PI-led but respond to strategic targets specified in the Roadmap and other planning documents. New Frontiers missions will enable aspects of the exploration of a range of objects, from Venus to giant planets, but will be limited in scope in terms of the complexity of operational capabilities at these bodies. Hence, they too will play key roles in solar system exploration but cannot achieve all of the measurement and exploration objectives necessary to answer the basic questions that motivate robotic exploration of the planets.
"Flagship-class" ($800 to 1400M or $1400 to $2800M) missions will be needed in order to reach and explore difficult but high priority targets. These critically important targets could help establish the limits of habitability, not just for our solar system, but for planetary systems in general. In particular, they potentially provide an opportunity to identify prebiotic organic molecules or even extant life beyond Earth, should it exist, in our own solar system. The targets of flagship missions include the surface of Venus, the lower atmosphere and surface of Titan, the surface and subsurface of Europa, the deep atmosphere of Neptune and the surface of its moon Triton, and the surface of a comet nucleus in the form of cryogenically preserved samples.
The next section discusses the program of missions and supporting research and technology development that will be necessary to answer the scientific questions posed above.
